The present disclosure relates to a semiconductor structure, and particularly to a method of locally tailoring the number of fingers in multi-finger fin field effect transistors, and structures formed by the same.
Fin field effect transistors (finFETs) provide advantage over conventional planar field effect transistors in terms of on-current per unit device area because sidewalls of semiconductor fins can be employed as the channel. In order to provide uniformity of widths among semiconductor fins, the semiconductor fins are formed as a dense one-dimensional array of semiconductor fins, and are subsequently patterned to provide a cluster of semiconductor fins laterally spaced from adjacent clusters of semiconductor fins.
Because the semiconductor fins have the same width and the same height in such cases, the on-current of a fin field effect transistor is determined by the number of semiconductor fins that a gate electrode straddles. Many semiconductor circuits, such as static random access memory circuits, require transistors having different on-currents. In such cases, the number of semiconductor fins in a single fin field effect transistor needs to be changed in order to provide transistors having different on-currents. Thus, it is necessary to locally cut semiconductor fins, i.e., to remove a portion of a long semiconductor fin in a region where a device with a reduced on-current needs to be formed.
Because the semiconductor fins are formed as a dense array, precise placement of a lithographic mask between semiconductor fins is a challenge, and causes significant level of reworks in lithographic processing and/or causes defective devices due to lithographic overlay problems, e.g., due to an unsuccessful cut or a cut that extends to a region where semiconductor fins should not be cut. Thus, a reliable method of cutting a semiconductor fin in a dense array of semiconductor fins is desired.
Because increasing the density of semiconductor fins enables greater layout density and reduced parasitic capacitance, fins are often patterned using sidewall image transfer processes, in which fins are located along the sidewalls of a mandrel shape. One property of the sidewall image transfer technique is that the number of fins formed is always even. Thus, the number of fins in a device may be increased or decreased by two by adding or removing a mandrel shape. However, an odd number of fins can only be formed by trimming. In this case, the ability to trim away a single fin is particularly important.